The venous system includes a multitude of one-way bicuspid valves that permit substantially unidirectional blood to flow toward the heart. These valves are particularly important in the lower extremities to prevent the pooling of blood. When the leaflets of the bicuspid valves fail to close properly, the valve is considered “incompetent” as it permits leakage of retrograde flow resulting in the abatement of flow towards the heart.
This potentially serious condition is known as “chronic venous insufficiency.” Symptoms can progress from unsightly “spider” or varicose veins to skin discoloration and painful skin ulcerations. The etiology of venous insufficiency is multifactorial, including a past history of thrombotic events, chronic venous stasis, and venous hypertension. Current treatments for venous insufficiency include elevation of the feet and compression stockings. While these can relieve symptoms, the underlying disease remains untreated. Surgical techniques are also employed in which native valves can be bypassed or replaced with autologous sections of veins having functioning valves.
Recently, various implantable medical devices and minimally invasive methods for implantation of these devices have been developed to deliver these medical devices within the lumen of a body vessel. These devices are advantageously inserted intravascularly, for example, from an implantation catheter. For example, implantable medical devices can function as a replacement venous valve, or restore native venous valve function by bringing incompetent valve leaflets into closer proximity. Such devices can comprise an expandable frame configured for implantation in the lumen of a body vessel, such as a vein. Venous valve devices can further comprise features that provide a valve function, such as opposable leaflets.
Dynamic fluctuations in the shape of the lumen of a vein pose challenges to the design of implantable prosthetic devices that conform to the interior shape of the lumen of a vein. Unlike arterial vessels, the flow velocity and diameter of veins does not remain essentially constant at a given systemic vascular resistance. Instead, the shape of vein lumens can fluctuate dynamically in response to the respiration, body position, central venous pressure, arterial inflow and calf muscle pump action of a mammalian subject. The veins also provide the principal volume capacitance organ. For example, an increase of almost 100% in the diameter of the common femoral vein has been observed in human patients simply by rotation of the patient by about 40 degrees, corresponding to a four-fold increase in blood flow volume. Moneta et al., “Duplex ultrasound assessment of venous diameters, peak velocities and flow patterns,” J. Vasc. Surg. 1988; 8; 286-291. Therefore, the shape of a lumen of a vein, which is substantially elliptical in cross-section, can undergo dramatic dynamic change as a result of varying blood flow velocities and volumes therethrough, presenting challenges for designing implantable intraluminal prosthetic devices that more closely conform to the changing shape of the vein lumen.
Current implantable devices for treating venous valve insufficiency may exhibit high radial force and be unresponsive to dynamic changes in the shape of a body vessel lumen, such as in a vein. For example, US2003/0055492A1 by Shaolian describes a self-expandable prosthetic venous valve mounted in a support structure exemplified by a plurality of longitudinally connected sinusoidal annular members. US2002/0099439A1 by Schwartz describes devices for radially compressing a vein proximate to an incompetent venous valve to improve or restore valve function. U.S. Pat. No. 6,494,909 to Greenhalgh describes a valve formed by a tube of braided filaments and a portion formed by a plurality of flexible leaflets, which may include fasteners positioned circumferentially around the tube to anchor the valve within the lumen of the vascular vessel. U.S. Pat. No. 6,482,228 to Norred describes an aortic heart valve which is adapted to be placed percutaneously and held in place with a stent system. U.S. Pat. No. 5,855,601 to Bessler describes an artificial heart valve having a stent member with a cylindrical shape and a flexible valve disposed in the stent member. Implantable prosthetic stents or valves for veins often have the same compressibility or expandability in any radial direction. Similarly, implantable device configurations with substantially invariant cross-sections upon implantation can be unresponsive to dynamic changes of the vessel cross-section, and can locally distort the shape of the body vessel.